Small, simple and stable lasers with a long term of life, that are spectrally pure and have a well defined optical frequency, are lacking today. Solid state lasers, e.g. In the shape of semiconductor elements that are equipped with an external cavity, would fulfill the requirements if they can be made small and stable. Such a laser can from a practical point of view be termed a fixed frequency laser.
A simple stable laser, with essentially corresponding practical properties as the fixed frequency laser, but having a wavelength that stays fixed in the environment of usage is also a desired object. Such a laser, which adapts its frequency in a fashion that the radiation wavelength remains constant, even when the atmospheric environment or some other surrounding gaseous environment varies, e.g. with respect to pressure and temperature, may be termed a fixed wavelength laser.
It may be of practical interest to let the term fixed laser comprise the two variants, the fixed frequency laser and the fixed wavelength laser. The prefix fixed implies that the frequency or wavelength from a practical point of view remains constant with respect to unintentional influences from the inner and outer environment, e.g. change in temperature, mechanical shock or change in humidity. The prefix has on the other hand no bearing on the possibility of controlling the frequency. Intentional alternation or modulation of the frequency is presupposed in various embodiments of the fixed laser, e.g. through geometrical alternation of the cavity of the laser device by forces from piezoelectric elements, through change of the temperature of the laser element etc.
In many applications it may suffice that the laser has a fixed frequency, but it may not be necessary that the frequency has a given value or that it can be altered. This may be the case, e g when the laser is used as an optical source for interferometric measurements in a stable outer environment, or when the requirements on measurement accuracy are moderate. Typical applications are measurements of geometrical dimensions or recordings of the change in position of an object, e.g. In manufacturing industry.
Another class of applications in which it may suffice that the fixed laser has a fixed frequency, that does not need to be altered, comprises usages of the laser as a local oscillator in measurements of the variations of the optical frequency of a laser light source.
Shall the laser on the other hand be used for accurate interferemetric measurements in an outer environment with varying air parameters then it As necessary to keep the air wavelength of the laser radiation constant, which implies that the frequency shall be adjusted along with the variations in the air parameters.
In many intended applications of the fixed laser it is necessary to be able to control its frequency. In spectroscopic applications of the fixed laser, e.g. In analysis of gases, one must be able to adjust its frequency to coincide with the frequency of an absorption line. Alteratively one must be able to sweep the frequency across a region that includes the absorption line.
Another field in which one must be able to control or modulate the frequency of the fixed laser, e g through changing the external cavity by piezoelectric or similar forces, is optical communication and other applications which Amply transmission of information.
To sum up, the fixed laser can be expected to find many applications as an optical source of radiation in applications concerned with measurements, analysis, sensing and information technology.
A realization of the fixed laser requires new solutions to a number of technical problems.
In conventional mechanical manufacturing and alignment of optical components in a laser device only a limited precision can be achieved. In terms of position and angular attitude this may typically be 10 .mu.m and 1 milliradian respectively. By using more costly manufacturing methods the precision can be increased somewhat. The tolerances mentioned are not sufficient to keep the external cavity adjusted for lasing, i.e. In a state that it can emit laser radiation. To achieve this, adjustments are required to an accuracy of a tenth or s few tenths of a .mu.m in position and a tenth or few tenths of a milliradian in angle.